Synthesis of Quaternary Amino Acids Bearing a (2‘Z)-Fluorovinyl α-Branch:  Potential PLP Enzyme Inactivators

Protected α-formyl amino acids, themselves available from the corresponding α-vinyl amino acids, are stereoselectively transformed into the (Z)-configured α-(2‘-fluoro)vinyl amino acids via a three-step sequence. The route employs McCarthy's reagent, diethyl α-fluoro-α-(phenylsulfonyl)methyl phosphonate, and proceeds via the intermediate (E)-α-fluorovinyl sulfones and (E)-α-fluorovinylstannanes. The latter may either be exploited as novel cross-coupling partners for fluorovinyl branch extension or be globally deprotected, to provide the title compounds.